Guide grooves of a recordable DVD (Digital Versatile Disc) are wobbled slightly, even without recording, in order to extract physical position information and a clock signal to be used for recording. A block configuration of a wobble signal extraction circuit of a conventional optical disc device is shown in FIG. 12. A laser beam hits the disc, the reflected light enters 4 light-receiving elements (tetrameric light-receiving element), and I-V (current-voltage) converted signals A-D are supplied to GCA (Gain Control Amplifier) circuits 2A-2D.
During a recording operation, marks are formed on the disc by means of recording light power, and unrecorded parts are left as spaces. Signals A-D contain signal elements (will be referred to as Write RF signal, hereinafter) for recording and wobble signal elements, and wobble elements with a relatively high C/N (Carrier/Noise) ratio are obtained from the reflected light generated by the light power.
These 4 signals A-D are configured in such a manner that said reflected light hits the guide grooves as sets (A+D) and (B+C), whereby a wobble signal can be obtained based on the difference between them. First, their amplitudes are adjusted coarsely by GCA circuits 2A-2D, (A+D) and (B+C) are computed subsequently by operational amplifiers 3A and 3B, fine gain adjustments are applied to them at GCA circuits 4A and 4B so as to optimize their gain balance, and the wobble elements are extracted by operational amplifier 5. In addition, the purpose of S/H (Sample-and-Hold) circuit 6 is to sample and hold the signal when the markers are formed, and it receives a timing signal that is required for generating the wobble signal elements from a light pulse block of sampling timing control circuit 9. This circuit shown in FIG. 12 generates an accurate wobble signal by means of a sampling method.
On the other hand, S/H circuit 6 is removed from the circuit shown in FIG. 12 in the circuit shown in FIG. 13, which is a block configuration of a circuit for generating a wobble signal by means of an averaging method that involves constant sampling.
In the circuits shown in FIG. 12 and FIG. 13, after only the frequency element required for a wobble signal is allowed to pass BPF (Band-Pass Filter: band-pass type filter) 7, automatic gain control is carried out by AGC (Automatic Gain Control) circuit 8 in order to output a signal with a prescribed amplitude. Here, the method described in Japanese Kokai Patent Application No. 2003-203352, for example, is available as a conventional example for detecting a wobble signal.
First, problems with the averaging method will be described. In the averaging method, the element necessary for control is extracted through averaging processing with the inclusion of the signal used for the marking/spacing, and S/H circuit 6 and sampling timing control circuit 9 shown in FIG. 12 are not required. Thus, an advantage is offered in that a timing does not have to be set for each velocity (write velocity) and each medium. However, as a result, in the case of a recordable disc, because the write power differs from the read power significantly especially during a recording, a significant difference is created in the amount of reflected light. Although significant modulation elements are also extracted from the wobble elements during the formation of the marks, only low-modulation elements are extracted during the formation of the spaces, resulting in the problem that the waveform is ultimately significantly distorted.
In addition, because the averaging method has the following disadvantages, the burden in terms of A/D conversion, correction, and restriction processing tends to increase in order to improve the wobble position detection capability and the clock precision during later-stage processing.                (1) The distortion is corrected by applying averaging processing to the respective extracted mark/space signals by BPF 7. In this case, however, because the modulation elements of the pre-averaged signals output from operational amplifier 5 return to almost zero at the spaces, the C/N ratio of the wobble signal falls more than one half as a result of the averaging processing.        (2) Wobble signal distortion and noise are increased at the mark-space switching points due to fluctuations in the characteristics among the light-receiving elements and the timing.        (3) A highly distorted signal is ultimately output due to mark-space level differences, and the elements become difficult to extract, particularly at the spaces. In addition, said distortion changes while the recording power is being changed so as to obtain the optimum value (OPC processing).        (4) As is the case with the DVD+R standard, when the wobble length and the RF signal pattern length approach each other, the 3 aforementioned points become more influential, and the quality of the wobble signal is ultimately degraded. Incidentally, in the case of a DVD-R, 6.7-31 Write RF (signal) patterns are generated during a half cycle of the wobble. On the other hand, in the case of a DVD+R, 1.1-5.3 are generated, and the RF signal pattern is close to the wobbling cycle.        
Next, problems with the sampling method will be described.
During the recording operation, the laser is driven pulsatingly in order to control the output of the write power and the read power. The write pulse generation block controls the pulse output required for each write based on a timing configuration table in order to output the laser at the optimum timing for the given RF signal pattern to be recorded. In addition to the control over the laser pulse, pulse output control for sampling an RF input signal for measurement control is also carried out. Sampling timing control circuit 9 shown in FIG. 12 is used to this end.
Although the reflected light from the medium is affected by the pulse signal, because the positions of the elements required for extracting the wobble signal contained in the reflected light are already known based on the mark-generating timing by sampling timing control circuit 9 in FIG. 12, adjustments are made in order for S/H circuit 6 to carry out the sampling at a stable level, except at the mark-space switching points.
The sampling timing is generated with reference to a recording clock generated from the wobble, and it needs to be set for each recording velocity, medium, and system. In this case, the system refers to the entire system that includes shifting of the timing caused by process fluctuations attributable to the light-reflecting characteristics of each disc, photosensor sensitivity, light-path formation precision, and the different chips used to configure control signals and signals to be controlled, wherein the more the respective elements change, the more difficult the adjustment of the timing.
In addition, when recording on a DVD, wherein the RF signal is configured at a constant linear velocity, at a constant angular velocity (for example, the CAV recording method), the recording clock needs to be follow with changes in the linear velocity, requiring a higher level of clock precision and complicated adjustments.
Sampling timing control circuit 9 is sometimes configured using a chip different from those used for the photodetector and GCA circuits (2A, 2B)-AGC circuit 8. In such case, a sample-and-hold control signal is supplied from the outside, and each chip will have 1 more pin. In addition, because the sampling pulse is propagated through the wiring on the substrate, there are risks of noise and unwanted radiation.